1.83 GA LATE-OROGENIC GRANITES OF SOUTHERN FINLAND: A LINK BETWEEN OROGENIC AND ANOROGENIC MAGMATISM IN THE FENNOSCANDIAN SHIELD

The Paleoproterozoic (Svecofennian) crustal domain in the central Fennoscandian shield was accreted onto a pre-existing Archean craton ca. 2.0-1.9 Ga ago in several magmatic events, early orogenic tonalitic and synorogenic continental arc-type magmatism in particular. Overall, these events have been envisioned as amalgamation of several separate terranes (microcontinents) with distinct origin and evolution. After the 1.88 Ga orogenic peak, contractional deformation continued in southernmost Finland where ca. 1.83 Ga old late-orogenic granites were emplaced as a 500-km-long, E-W-trending belt characterized by migmatization with granitic leucosome. The late-orogenic granites vary from relatively large, homogeneous, coarse-grained, locally porphyritic plutons to small intrusions of equigranular granite. They are typically peraluminous and are also found as granitic dike systems migmatizing the supracrustal rocks. Nd isotopic data indicate highly variable initial compositions with epsilon-Nd (at 1.83 Ga) values of -6.5 to +2.2. This suggests that the late-orogenic granite belt transects at least three Paleoproterozoic terrane boundaries. There is no generally accepted conception of whether the late-orogenic granites were emplaced during transpressional deformation or during extension related to orogenic collapse. They do, however, provide a temporal and spatial link between the orogenic (1.91-1.87 Ga) and anorogenic (1.67-1.54 Ga) magmatism in southern Finland, and delineate a tectonic domain that subsequently produced vast volumes of anorogenic rapakivi granite.